The present invention relates generally to cables for transmitting information and, more particularly, to a conduit for injection of a compound into the interior of electrical cables.
Underground electrical cable technology was developed and implemented because of its aesthetic advantages and reliability. Currently, underground electrical cables generally include a number of copper or aluminum strands surrounded by a semiconducting or insulating strand shield, a layer of insulation, and an insulation shield.
Underground electrical cables were initially touted as having a useful life of 25-40 years. However, the useful life of such cables has rarely exceeded 20 years, and has occasionally been as short as 10-12 years. In particular, the insulation tends to degrade over time because water enters the cable and forms water trees. Water trees are formed in the insulation when medium to high voltage alternating current is applied to a polymeric dielectric (insulator) in the presence of water and ions. As water trees grow, they compromise the dielectric properties of the polymer until the insulation fails. Many large water trees initiate at the site of an imperfection or a contaminant, but contamination is not a necessary condition for water trees to propagate.
Water tree growth can be eliminated or retarded by removing or minimizing the water or ions, or by reducing the voltage stress. Voltage stress can be minimized by employing thicker insulation. Clean room manufacturing processes can be used to both eliminate ion sources and minimize defects or contaminants that function as tree growth sites. Another approach is to change the character of the dielectric insulator, either through adding water tree retardant chemicals to the insulator, or by using more expensive, but water tree resistant, plastics or rubbers. Still yet another approach to eliminate or retard water tree growth is to encapsulate the entire electrical cable within a conduit having a larger diameter than the electrical cable. All of these approaches have merit, but only address the performance of electrical cable yet to be installed.
For electrical cables already underground, the options are more limited. Currently, a dielectric enhancement fluid may be injected into the interstices between the strands of electrical cables. The dielectric enhancement fluid reacts with water in the underground cable and polymerizes to form a water tree retardant that is more advanced than those used in the manufacture of modem cables. Although the injection of a dielectric enhancement fluid into the interstices of an electrical cable is effective as a water tree retardant, it is not without its problems.
First, the interstices between the strands of the cable may be blocked for a variety of reasons, including the presence of a splice, strand blocking material, or because the strands are highly compacted. As a result, it is often difficult, if not impossible, to inject the dielectric enhancement fluid into the cable. Second, in certain cables having a relatively small diameter, such as underground residential distribution (URD) cables, there is not enough interstitial volume between the strands of the cable to hold sufficient amounts of the dielectric enhancement fluid for maximum dielectric performance. As a result, such cables require an extended soak period of 60 days or more to allow for additional dielectric enhancement fluid to diffuse from the cable strands into the insulation layer. Finally, encapsulating an entire cable within a conduit is expensive.
Thus, there exists a need for a flow-through cable for transmitting information in which a compound can be injected into and distributed throughout, the cable at a relatively low cost, a high degree of reliability, and without interrupting the flow of current through the cable.
In accordance with the present invention, a flow-through cable for transmitting information is provided. The cable includes an information conducting core. The cable also includes a first insulation layer surrounding the information conducting core and a first conduit disposed within either the information conducting core or the first insulation layer. The first conduit is adapted to permit a compound flow therethrough. The first conduit is chemically permeable to permit at least a portion of the compound to diffuse through the first conduit and into the first insulation layer.
In accordance with other aspects of this invention, the information conducting core is a plurality of power strands.
In accordance with additional aspects of this invention, the first conduit is centrally received within the plurality of power strands. In accordance with other aspects of this invention, the cable further includes a chemically permeable second conduit, wherein the first and second conduits are disposed within the plurality of power strands.
In accordance with still yet other aspects of this invention, the cable further includes a strand shield surrounding the plurality of power strands, and the first and second conduits are disposed within the strand shield.
A flow-through cable for transmitting information formed in accordance with the present invention has several advantages over electric cables used in the past. First, disposing a first chemically permeable conduit within the cable eliminates the expense of sheathing the power cable within a large conduit. Second, providing a dedicated conduit to distribute a restoration compound throughout the length of a cable ensures an unblocked path through which the restoration compound may flow-through the entire length of the cable. Further, because the chemically permeable conduit is adapted to receive a variety of compounds, such as a desiccant liquid, gas or a tracer fluid, a flow-through cable for transmitting information formed in accordance with the present invention is more robust than those currently available. In summary, a flow-through cable for transmitting information formed in accordance with the present invention is cheaper to maintain and operate, more reliable, and more robust than currently available electric cables.